This invention relates to the field of organic chemistry, and more particularly, to an improvement in the process for the preparation of p-aminophenol from nitrobenzene.
p-Aminophenol (PAP) is an important chemical intermediate used in the preparation of the analgesic, acetaminophen. A number of other derivatives having a wide variety of industrial applications may also be prepared from PAP. One method for preparing PAP involves the catalytic hydrogenation of nitrobenzene in an acid medium. In this process, phenylhydroxylamine is first formed, and this intermediate promptly rearranges in the acid reaction medium to form PAP. This rearrangement of phenylhydroxylamine in acid solution was first described in two 1894 reports by Bamberger (Ber. 27, 1347 and 1548). While the desired reaction proceeds as indicated, a competing second reation takes place to a varying extent, resulting in the production of aniline, rather than PAP. Lesser amounts of other by-products are also formed.
Henke, et al. (U.S. Pat. No. 2,198,249) were the first to disclose a process for the preparation of PAP by the catalytic hydrogenation of nitrobenzene in an acid medium.
A number of improvements have since been reported. For example, Spiegler (U.S. Pat. No. 2,765,342) studied the reaction extensively. Among the factors explored by Spiegler was the effect on reduction rate and p-aminophenol yield of including a surfactant selected from among several quaternary ammonium compounds and several non-quaternary compounds. Among the quaternary ammonium compounds used was dodecyl trimethylammonium chloride; the non-quaternary compounds investigated included two simple tertiary amine salts: triethylamine sulfate and tributylamine sulfate, as well as dioctadecyl propyleneamine dioleate. From a plot of rate and yield data, Spiegler concluded that the rate/yield performance of all of the quaternary ammonium compounds examined was superior to that of the non-quaternary compounds.
Later, Brown et al. (U.S. Pat. No. 3,535,382) reported that certain nonionic polyether polyol surfactants could be substituted for Spiegler's quaternary ammonium compounds.
R. G. Benner (U.S. Pat. No. 3,383,416) used the Henke et al. approach of charging all the nitrobenzene at once, but used a carbon-supported platinum catalyst and quaternary ammonium surfactant, preferably dodecyl trimethylammonium chloride, as disclosed by Spiegler. Benner purposely interrupted the hydrogenation well before all the nitrobenzene had been consumed. In the presence of two liquid phases, aqueous and nitrobenzene, a carbon-supported platinum catalyst is preferentially wetted by the nitrobenzene, so most of the catalyst is suspended in the nitrobenzene phase. Thus, when hydrogenation is interrupted while there is still a distinct nitrobenzene phase, the catalyst preferentially remains suspended in the lower nitrobenzene layer, permitting the removal of the upper aqueous solution of PAP and aniline by decantation. The PAP is then recovered from the aqueous layer and purified.
One method of recovering the PAP involves neutralizing the aqueous phase to precipitate the PAP. At the same time, the aniline, which is dissolved as its salt in the acid solution, separates as an organic phase in the neutral solution and is readily separated from the aqueous phase.
Although PAP is the principal product of the process discussed above, the by-product aniline is also of commercial significance. While the relative yields of PAP and aniline are sensitive to a number of variables, such as hydrogen pressure, acidity, catalyst, temperature, surfactant, degree of agitation, etc., the by-product aniline yield will often amount to 10 to 25% or more of the yield of PAP. Thus, the marketability of the by-product aniline is a significant factor in the overall economy of the PAP process.
It has been found that one drawback to the use of trimethyldodecylammonium chloride as a surfactant in the PAP-from-nitrobenzene process is that it leads to undesirable minor by-products in the aniline phase. First of all, the chloride introduced in the quaternary ammonium salt leads to the formation of small amounts of o and p-chloroaniline, respectively. Morever, the form in which the quaternary ammonium salt is supplied commercially is a solution in water/isopropyl alcohol. The isopropyl alcohol thus introduced into the process with the quaternary salt leads to the formation of small amounts of o and p-isopropoxyaniline, respectively. These chloro- and isopropoxyaniline impurities follow the aniline in the separation steps and collectively appear to the extent of about 1% in the crude aniline thus recovered. Since by-product aniline containing these impurities must compete in the market place with aniline from other sources which does not contain these impurities, their presence in the aniline is a drawback.
Accordingly, the use in the PAP-from-nitrobenzene process of a process additive which possesses the useful characteristics of trimethyldodecylammonium chloride, but which avoids the production of chloro- and isopropoxyanilines would be desirable. It is an object of this invention to define such an additive. Other objects will be in part apparent and in part pointed out hereinafter.